[DependencyInjection] Enhance tests for class Container
minor #11762 [DependencyInjection] Enhance tests for class Container (kerdany)

This PR was squashed before being merged into the 2.6-dev branch (closes #11762).

Discussion
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[DependencyInjection] Enhance tests for class Container

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

Added missing tests/assertions:
- Assertions that ->compile() resolves parameter bag.
- Asserton that ->getServiceIds() shows ids defined by set() after ids defined by get*Service() methods.
- Test that ->set() automatically calls synchronize*Service if defined. Updated associated fixtures in the ProjectServiceContainer class definition.
- Assertion that ->get() is case insensitive.
- Assertion that leaving an inner scope with active child scope, to an outer scope, deactivates/resets the child scope(s).
- Test that entering a child scope recursively resets the inner scope.
- Test that a scope can not be entered before it's added first.
- Test that a scope can not be entered before adding and entering the parent scope first (for non container scopes).
- Test for underscore().

Other changes:
- Added missing messages in some assertions.
- Moved testGetThrowsException*() methods close to testGet*() tests.
- Renamed variable(s) '$services' (referencing the $sc->scopedServices field) to $scoped, so as not to confuse it with the global scope map.
- Minor refactoring in class Container for code consistency and reducing redundancy.

Commits
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6211205 [DependencyInjection] Enhance tests for class Container
Split of the SecurityContext and Interface
Fixed unit-tests
Added Security deprecations/changes to UPDGRADE to 2.6